Electropolishing steel



United States Patent ELECTROPOLISHING STEEL Christian J. Wernlund, Niagara Falls, N.Y., assignor to E. L du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Jan. 17, 1958, Ser. No. 709,469

4 Claims. (Cl. 204-48) This invention relates to the electropolishing of ferrous surfaces by treatment in an improved electrolyte.

In the metal finishing industry it is generally desirable to give metal articles smooth polished surfaces. This is especially true of surfaces to be coated with protective or decorative plates such as nickel, copper, chromium and the like. Various electrolytic polishing processes, generally consisting of anodic treatment in acidic solutions, have been proposed for electropolishing steel and some of these have practical utility for limited specific purposes. However, such anodic processes are especially inadequate for the treatment of unalloyed carbon steels such as are used in automobile parts and the like which require surface coatings such as electroplates.

Among the problems associated with present commercial acid baths for the anodic treatment of low carbon steels, there is the fact that insoluble iron sludge, for example iron sulfate, forms in the bath and this raises more or less serious disposal problems. Other shortcomings relate to inadequate surface leveling and smut formation. But, above all, these baths are not good surface brighteners.

It is, accordingly, a major objective of the present invention to provide a bath which will combine electropolishing brightening and deburring with surface leveling by anodically treating low carbon steels while at the same time depositing the anodically dissolved iron on the cathode as a useful pure iron product. Other objects will appear from the further discussion of the invention.

In accordance with my invention, I have discovered that low carbon steel articles can be anodically treated in an aqueous bath containing iron fiuoborate and fluoboric acid in such a way as to brighten and smooth the steel surface while at the same time depositing dissolved iron on the cathode.

In my process the article to be polished is made the anode in a solution containing iron fiuoborate and fiuoboric acid. The cathode may be any metal such as iron, lead, copper or nickel. The current density is adjusted so that a high single potential is set up at the anode surface which will practically eliminate all intercrystalline attack on the steel such as is experienced whenever iron is dissolved anodically in any strong acid such as a sulfuric acid solution. A voltage from about 4 to about 15 volts is required and this will establish an anode current density range from about 60 ampere per square foot to about 300 ampere per square foot. The temperature range of the electrolyte should be below about 40 C. and preferably between about 15 to 25 C. for good, consistent smooth electropolishing of low carbon steels. In general, a time between about 1 and minutes is, required to secure a proper anodization of the iron surface.

The following record of experiments will further amplify the invention:

' Example 1 A small cell fitted with two steel cathodes was set up to receive an anode sheet about 7 inch thick, of low carbon steel so that an area of /2" by 3" was submerged in the electrolyte about equidistant between the two cathodes which were about 2 inches apart. The electrolyte consisted of 200 ml. of a 48% solution of fluoboric acid, HBR; which is approximately equal to 540 g. per liter of HBF and of a sp. gr. of 1.190. A direct current of about 4.0 volts and at a current density of about 384 amp/sq. ft. was passed through the cell for 3 minutes at a temperature between 25 and 27 C. The anodically treated steel sheet had lost 1.07 mils of iron and the electrolyte contained the equivalent of 8.4 g./1. of iron fiuoborate, Fe(BF The treated surface was smooth and bright and excellently deburred.

Example 2 This experiment was a continuation of Example 1 except that a new anode sheet of the same type of low carbon steel was inserted and electrolysis continued for 4 minutes at 22-23 C. and a current density of 288 amps/sq. ft. at 6 volts. At the end of this treatment the bath contained about 16.2 g./l. of iron fiuoborate. The steel sheet had lost about 1 mil in thickness and the surface was smooth and bright with excellent deburring;

Example 3 This experiment was a continuation of the previous example except that a new anode sheet of the same type of steel was inserted in the electrolyte and subjected to anodic treatment for 3 minutes at 22 C. at a voltage between 2.9 and 3.3 volts and a current density of about 240 amps/sq. ft. The anodically treated surface was smooth and bright with good deburring. At the end of this period, the electrolyte contained the equavalent of 22.4 grams/liter of iron fiuoborate.

Example 4 Example 5 This experiment was a repetition of the previous example in the same electrolyte with a new sheet of steel for the anodic treatment. The temperature and the time of treatment was the same, but the voltage was about 1.5 with a current density of about 156 amps/sq. ft. The etch was again rough and dull with poor deburring, showing that low voltage must be avoided.

Example 6 This experiment was conducted in the same electrolyte as the previous example except that the electrolyte now contained 39.7 g./l. of iron fiuoborate. The temperature of anodic treatment was 21-23 C. and the time 8 minutes. The voltage varied between 3.7 to 4.0 and the anode amperage/sq. ft. 220 to 230. Nearly 2 mils of metal had been removed from the anode surface which was bright and very smooth with good deburring.

Example 7 A new sheet of low carbon steel was inserted in the electrolytev from the preceding example. The anodic. treatment lasted for 4 minutes at 22-24 C. at 4.3 to 4.5 volts with anode amperage of about 266 amps./sq. ft. The anode was reduced in thickness by a little over 1 mil and the treated surface showed good leveling anti 3 deburring as well as brightness. At the end of the treatment the anolyte contained about 62.5 g./l. of iron fluoborate. I

. Example 8 The electrolyte from the previous example was used to treat anodically a sheet of low carbon steel in the same apparatus. After 57'minutes at 3.7 to 4.2 volts and 200-220 amperes per square foot the bath contained 287.8 g./l. of Fe(BF The sheet had lost about 28 mils in thickness. The anode current efiiciency in dissolving iron was about 98.8% of theory. The sheet was bright etched, very smooth and clean. This experiment shows that the iron fiuoborate-fluoboric acid bath may be used to deep etch or chemically mill a steel surface with a resultant smooth, even-leveled surface, no matter how much iron is removed.

Example 9 An iron drill rod /2" diam. and 5.25 long was subjected to anodic electrolysis for 60 minutes in a 2 liter electrolyte containing by weight about 52% Fe(BF 2.3% Fe(BF 14.7% HBF and the balance water. The rod was blanked oil by adherent insulation so as to leave a 2" section exposed in the electrolyte at a V2" distance from the lower end. Temperature of the electrolyte varied between26 to 30 C.; the voltage was between 10.3 and 10.6 at 600 to 650 amperes per square foot. The exposed 2 portion of the rod was reduced 0.054" or 54 mils in diameter. The smoothly etched bright iron surface resembled a smooth machined hearing surface. This experiment demonstrates that the ferrous fiuoborate-fluoboric acid is suitable for anodic or chemical milling of low carbon steels.

Example An electrolytic bath which had been used in the anodic treatment of numerous samples of low carbon steels had reached substantial equilibrium in composition such that it contained by weight about 55.6% Fe(BF 0.4% Fe(BF 10.8% HBE, with the balance water and other materials in very minor proportion. Two iron cathodes plated with copper were suspended in this electrolyte with a combined effective electrolytic area of 16 inches. Between these cathodes then was placed a sheet of low carbon steel exposing 4 sq. in. to anodic treatment. After minutes of electrolysis at a bath temperature of 23 C. and a voltage varying between 3 and 4.2 and an approximate 108 amperes per square foot the anode lost 0.760 g. in weight and the cathodes gained 0.670 g. The anode surface was rendered smooth and bright with a whitish sheen. The cathodes were plated with bright Example 13 The electrolyte of the previous example was used except that one gram of a wetting agent, 2-naphthol-8- sulfonic acid was added per liter of the solution. One anode treated for 5 minutes at 5.3 to 7.8 volts and 96 to 113 amperes per square foot at a temperature between 2027 C. reduced the R.M.S from 22 to 10. Another anode treated in the same bath for 10 minutes at 18 to 29 C., the same amperage and voltage between 5.5 to 8.2 resulted in an R.M.S. reduction'from 24 to 9. in general, it was found that the presence of a wetting agent improved the reduction of the surface roughness over that in which no wetting agent was used by 25 to 50%. Y

Fluoboric acid is the etchant agent in the electro leveling process of this invention. However, the fiuoboric acid aqueous medium has the desirable capability of holding a large amountof iron fluoborate in solution up to a concentration range at which an amount of iron is deposited practically equivalent to the amount removed from the cathode. This makes possible the use of this new bath for continuous operation without the build-up iron which exfoliated on the sides adjacent to the anode.

Example 11 A sheet of steel was anodized between two cathodes as defined in Example 1. The anode surface treated was 3 sq. in. The temperature varied from 12 to 22 C. over the anodizing time of 12 minutes and the voltage varied from 5.5 to 6.5 with amperage of 105 to 126 a./s.f. The anodized sheet lost about 1.14 mils in thickness. The bath was made by dissolving 100 grams of iron power (by hydrogen) in 765 g. fiuoboric acid (48% HBF aqueous solution. The R.M.S. (root mean square) surface roughness in micro inches as determined by a conventional brush surface analyzer was 24 for the untreated surface and 12 for the anodized surface, showing a very significant roughness improvement.

Example 12 q The electrolyte from Example 11 was used. Temperature of the bath was between 14 and ;l6 C. and the current was applied for 16 minutes at 4.5 to 7.1 volts with about 72 amperes per square foot. The anode lost 1.08 mils in thickness and the R.M.S. was reduced from 27 to 14.

of large amounts of insoluble iron containing sludges which require removal to keep the bath in operation in other conventional etching baths. The amount of ferrous fiuoborate that will be present in solution under equilibrium conditions will vary from about 40% to 60% and the fiuoboric acid from 0.5% to 48%. There will also usually be present some ferric fluoborate varying with conditions conducive to oxidation of ferrous iron to the ferric state. An amount up to about 10% of ferric fiuoborate will not be deleterious in the electrolyte. It has been found that the fiuoboric acid-iron fluoborate must contain some Water but eflfective electrolytic surface treatment will not take place if morethan 50% by weight of the electrolyte is water. A water content between 10% and 40% is the preferred range.

Agitation of the electrolyte may be advantageous when high speed etching is desired but otherwise agitation is of secondary importance.

High current densities are generally desirable provided only that anode passivity is avoided. Current efficiencies at both the anode and the cathode will approach theoretical values as optimum concentration of fiuoboric acid and iron fluoborate is maintained.

The fiuoboric acid-iron fiuoborate bath is very effective in conducting chemical milling operations on low carbon steels. The amount of iron removed will depend upon the length of electrolysis and the amperage. The milled surface, in general, will be very smooth and clean.

It has been found that the anodically cleaned surfaces resulting from the practice of this invention are more resistant to corrosion than the untreated surfaces. These treated surfaces also constitute good bases for applying other metal electrodeposits.

There will gradually accumulate a sludge in the fluoborate-fluoboric acid electrolytic etching bath consisting of impurities coming from the iron subjected to treatment. This sludge consists largely of carbon and silica and can be readily removed by simple decantation or filtering operations and the bath is reusable.

I claim:

1. The process for electropolishing ferrous articles which comprises anodically removing iron from said article and depositing iron on the cathode in an electrolyte containing by weight between about l0% and 40% water, 0.5 to 48% fiuoboric acid and the balance iron fluoborate.

2. The process for electropolishing ferrous articles which comprises anodically treating said articles in an electrolyte consisting essentially of at least i0% ,but not more than 50% by weight of water, 0.5 to 48% by weight of fiuoboric acid, and the balance iron fluoborate, and simultaneously depositing iron from the electrolyte on thecathode. J

3. The process for electropolishing ferrous articles which comprises anodically treating said articles in an electrolyte consisting essentially of at least 10% but not more than 50% by weight of water, 0.5 to 48% by weight of fluoboric acid, and the balance iron fluoborate.

4. The process for electropolishing ferrous articles which comprises anodically removing iron from said article in an electrolyte containing by weight between about 10% and 40% water, 0.5 to 48% fluoboric acid and the balance iron fluoborate.

References Cited in the file of this patent UNITED STATES PATENTS Mason Feb. 15, 1938 Guthrie Apr. 4, 1938 Moir Mar. 28, 1950 Struyk et a1. Sept. 19, 1950 Neill Feb. 20, 1951 Poor May '15, 1956 OTHER REFERENCES Metal Finishing, September 1951, pp. 86-87. 

1. THE PROCESS FOR ELECTROPOLISHING FERROUS ARTICLES WHICH COMPRISES ANODICALLY REMOVING IRON FROM SAID ARTICLE AND DEPOSITING IRON ON THE CATHODE IN AN ELECTROLYTE CONTAINING BY WEIGHT BETWEEN ABOUT 10% AND 40% WATER, 0.5% TO 48% FLUOBORIC ACID AND THE BALANCE IRON FLUOBORATE. 